In recent years, a silicone hydrogel has been known as a material of a contact lens for use in continuous wear. The silicone hydrogel is obtained by combining a silicone component with a hydrophilic component, and as one example thereof is known a silicone hydrogel obtained by polymerizing a polymerization mixture containing a silicone acrylamide monomer, and a hydrophilic acrylamide monomer, a hydrophilic methacrylate and an internal wetting agent for imparting wettability to a surface (U.S. Pat. No. 7,396,890 and U.S. Pat. No. 7,214,809).
However, the compositions described in U.S. Pat. No. 7,396,890 and U.S. Pat. No. 7,214,809 have at least one methacrylate as a component in a relatively large amount. The resulting mixed acrylate/methacrylamide systems display a reduction in the polymerization rate of the system compared to an all acrylamide system.
Japanese Patent Laid-Open No. H10-212355 and U.S. Pat. No. 4,711,943 disclose silicone hydrogels comprised of a silicone acrylamide monomer and a hydrophilic acrylamide monomer. Such silicone hydrogels have compositions mostly constituted by acrylamide monomers, and improvement of the polymerization rate of the whole system is expected. However, crosslinkers used in these documents have methacrylate group such as ethylene glycol dimethacrylate or tetraethylene glycol dimethacrylate. When such crosslinkers are copolymerized with a (meth)acrylamide monomer, the crosslinker is consumed early in the polymerization. This can lead to a heterogenous polymer network and optical distortion may occur when a contact lens is produced using such a polymer.
U.S. Pat. No. 4,711,943 also discloses a composition using a silicone bisacrylamide monomer as a main component of the polymer. Lenses formed from these monomers are hard, and wearer comfort is decreased.
Silicone containing bis(meth)acrylamide monomers having two (meth)acrylamide groups have also been disclosed.
WO 2010/147874 discloses Ma2D37 having a silicone segment with 37 repeating units in a silicone bis(meth)acrylamide monomer. However, the only silicone component disclosed to be polymerized with the silicon bis(meth)acrylamide is a mono-functional branched silicone (meth)acrylamide monomer. The resulting polymer displayed poor shape recovery. The composition disclosed in WO 2010/147874 also contains N-vinylpyrrolidone in a relatively large amount, which negatively effects the polymerization rate as well.
WO2010/071691 discloses silicone bis(meth)acrylamide monomers having molecular weights of 4500 and 11000. WO2010/071691 also discloses that it is critical to include a silicone-containing vinylic monomer having a tris(trialkylsilyloxy)silylalkyl group to eliminate optical defects. The polymers are disclosed to exhibit a “healing” effect (folding marks become transient), however, “shape recovery”, or the ability of a polymer to recover tensile strength after being released from polymer elongation, is not disclosed. Shape recovery is evaluated by measuring stress zero time. Stress zero time of the polymers in the reference is expected to be longer (worse shape recovery) than those in the present invention because mono-functional silicone acrylamide is not linear silicone.
US2011/0009519 discloses Ma2D37 as a silicone bis(meth)acrylamide monomer. Various monomers are polymerized with Ma2D37, but none are mono-functional (meth) acrylamide silicone monomers as presently disclosed. The closest monomer has —Si—CH2-CH2-Si— bond alternating with the —Si—O—Si— bonds and an undesirably high modulus.
European Patent No. 1956033 discloses a silicone bisacrylamide monomer in Synthesis Example 10. However, this monomer is only used as an intermediate for synthesizing a silicone tetraacrylamide compound in Example 10, and there are no descriptions of copolymerization with other silicone monomers, the modulus, transparency or any other properties of a copolymer obtained thereby.
N,N′-methylenebisacrylamide (hereinafter referred to as MBA) is a commercially available acrylamide crosslinker. Unfortunately when MBA is used as a crosslinker, undesirably high moduli are obtained. When the amount of crosslinker is decreased, the modulus is reduced, but the copolymer loses transparency before the modulus is sufficiently reduced, thus making it difficult to have both the low modulus and transparency. Furthermore, when an attempt was made to reduce the modulus by using various bisacrylamides having extended methylene chains, there was no significant difference in modulus compared to MBA, and when the amount of crosslinker was decreased, the transparency was reduced, thus making it difficult to have both low modulus and transparency.